Termite mounds contain distinct methanotroph communities that are kinetically adapted to elevated methane concentrations

Abstract

20Termite mounds have recently been confirmed to mitigate approximately half of termite 21 methane (CH4) emissions, but the aerobic methane-oxidizing bacteria (methanotrophs) 22 responsible for this consumption have not been resolved. Here we describe the 23 abundance, composition, and kinetics of the methanotroph communities in the mounds 24 of three distinct termite species. We show that methanotrophs are rare members of the 25 termite mound biosphere and have a comparable abundance, but distinct composition, to 26 those of adjoining soil samples. Across all mounds, the most abundant and prevalent 27 particulate methane monooxygenase sequences detected were affiliated with Upland Soil 28Cluster α (USCα), with sequences homologous to Methylocystis and Tropical Upland Soil 29Cluster also detected. The Michaelis-Menten kinetics of CH4 oxidation in mounds were 30 estimated from in situ reaction rates. The apparent CH4 affinities of the communities were 31 in the low micromolar range, which is one to two orders of magnitude higher than those 32 of upland soils, but significantly lower than those measured in soils with a large CH4 33 source such as landfill-cover soils. The rate constant of CH4 oxidation, as well as the 34 porosity of the mound material, were significantly positively correlated with the abundance 35 of methanotroph communities of termite mounds. We conclude that termite-derived CH4 36 emissions have selected for unique methanotroph communities that are kinetically 37 adapted to elevated CH4 concentrations. However, factors other than substrate 38 concentration appear to limit methanotroph abundance and hence these bacteria only 39 partially mitigate termite-derived CH4 emissions. Our results also highlight the 40 predominant role of USCα in an environment with elevated CH4 concentrations and 41 suggest a higher functional diversity within this group than previously recognised. 42 43 5 to 10 cm of the mound. For a subset of the investigated mounds, soil was collected from 132

The role of termite CH<sub>4</sub> emissions on ecosystem scale: a case study in the Amazon rain forest

“…Methanotrophic bacteria are responsible for the CH 4 uptake in aerobic soils, and their possible presence in termite mounds was already suggested by Seiler et al (1984). Other studies have confirmed their presence (Chiri et al, 2019;Ho et al, 2013), and recent studies have been focusing on whether https://doi.org/10.5194/bg-2020-384 Preprint. Discussion started: 16 November 2020 c Author(s) 2020.…”

“…Other estimates range widely between no observable uptake to very strong uptake rates (up to 80%) (Khalil et al, 1990;Macdonald et al, 1998;Nauer et al, 2018;Sugimoto et al, 1998a). A more elaborate overview of recent findings on termite mound CH 4 uptake processes can be found in Nauer et al (2018) and Chiri et al (2019). The role of possible mound CH 4 uptake should also be acknowledged for the measurement of individual termite emissions (Table 2, upper part): most literature values, including values from this study, are based on termite incubation in presence of mound material, with ongoing CH 4 uptake, wherefore actual termite CH 4 emission values might be higher.…”

Bark-dwelling methanotrophic bacteria decrease methane emissions from trees

“…The V4 hypervariable region of the 16S rRNA gene was ampli ed using the universal Earth Microbiome Project primer pairs 515FB 5′-GTGYCAGCMGCCGCGGTAA-3′ and 806RB 5′-GGACTACNVGGGTWTCTAAT-3′ 57 , as per previously described cycling conditions 58 . The employed reaction conditions and thermal pro les of the qPCR assays have been previously described 36 . Ampli cation from different dilutions (from undiluted to 1:100 dilution in PCR-grade water) of DNA extracts was tested, and the dilution resulting in the highest yield and quality of PCR product was used for the qPCR assays.…”

“…Note that this analysis cannot detect uncultured MOB with unknown 16S rRNA gene sequences. Data processing of the pmoA gene amplicon sequences followed our previously published pipeline 36,61 , with minor modi cations. All processing steps were performed in the QIIME 2 platform and, instead of assigning the raw sequences to operational taxonomic units, raw sequences were denoised using the DADA2 pipeline 62 , yielding 280 high-quality pmoA amplicon sequence variants (pmoA-ASVs).…”

“…Here, we establish that tree stem bark can host a previously uncharacterised, novel microbiome and unique MOB community, which may substantially mitigate tree stem methane emissions and thereby help regulate Earth's climate. Our study combined the use carbon stable isotope analysis 23,27,32,33 , in situ methanotrophy inhibitors 34,35 , and molecular community pro ling 36,37 , which have each been previously used to determine the rates and mediators of microbial oxidation in wetlands and other environments. On this basis, we provide multiple lines of biogeochemical and microbial evidence that abundant MOB occupy tree bark and represent a novel methane sink.…”